The early development of the cellular components of the lympho-myeloid complex in the leopard frog, Rana pipiens is being investigated. The basic experimental approach is the reciprocal transplantation of presumptive hematopoietic anlagen between diploid (2N) and triploid (3N) frog embryos. The cellular components of hematopoietic organs in experimental larvae are subsequently analyzed using microdensitometric methodologies in order to distinguish the embryonic origin of the particular components. Data obtained using this experimental approach have shown the hemopoietic stem cells originate from dorsal lateral plate mesoderm in the region of the presumptive mesonephros. These precursor cells migrate along the developing pronephric duct and subsequently invade the dorsal aortae and posterior cardinal veins at a time coincident with the initiation of circulation.